experimented a bit more, and seem to have something that's working a bit better. However, not a lot of dps gain.

Changed the level 3 talent, and went with Mindbender instead, so the macro looks like this:

Code:
/stopmacro [channeling]
/castsequence Shadow Word: Pain,Vampiric Touch,Mind Blast
/cast Mind Flay
/cast Mind Flay
/cast Mind Flay
/cast Devouring Plague
/cast Mindbender
This DOES progress through everything, but I haven't worked out yet what things need to be cast more often. It would appear that SW:P and Vamp are still being refreshed too frequently, but all the mechanics are working.

I'm also staying at near max mana with this, but I'm not sure if that's a result of solo-targeting, or not using enough mana intensive dps spells.